There doesn't seem to be a way to get a guild timeline - who the charter members were, when various toons came and went (obviously to the precision of available data). Has anyone figured out how to find this information?

Well, the problem with what you want to do is that there was never a requirement that charter members all had to be online during or directly after the creation... In fact, with the previous need of 9 signatures, many guilds took a fair while to establish and most were created with one or more charter members offline.

As for comings and goings... that's partially doable, but a lot manual work.

First, you need to visit every listed member and check the first date his/her data lists membership in the guild you are interested in.

Second, you check armory for all. Those that still exist will show their current guild. If it matches the data on character's WCR entry, that character is ok. If not, WCR is missing data and date of going can't be determined accurately.

If the character is not found on Armory, it has name, faction and/or realm changed... or has been deleted and again, date of going can't be determined accurately.

Second phase, do a similar check on every single Ex-member character.

Examine each character's guild membership data, noting down the first date he/she was seen. If this date matches another "unguilded" or member of another guild date, you have an exact date of entry. If the "unguilded" or member of another guild date is the previous day, the entry date might still be accurate. If there is a bigger gap between previous entry and the date found, then the entry point falls somewhere in between, with no actual way of being sure of exact date.

Exit check follows a similar pattern:

If the character has been seen as "unguilded" or member of another guild during the same day that he/she was last seen in the guild you are interested in, you have an exact date. If the next entry is the following day, the date might still be accurate. If the gap is bigger, there is no way of determining exact date of leaving.

Sorry that I could not help much, but a lot of the data you would like simply isn't available, in fact, some of it would probably be hard, possibly impossible for even Blizzard itself to piece together after a sufficiently long time has passed.